Variable displacement pumps are commonly used to provide adjustable fluid flows to machine actuators, for example to cylinders or motors associated with moving machine tools or linkage. Based on a demand of the actuators, the displacement of the pump is either increased or decreased such that the actuators move the tools and/or linkage at an expected speed and/or with an expected force. Historically, the displacement of the pump has been controlled by way of load-sensing, pilot-type valves that are connected to a displacement actuator of the pump.
Although adequate for some situations, pilot-type valves can be slow to respond and inaccurate. That is, because the valves are hydraulically moved by a difference between a desired pressure and an actual pressure acting directly on the valves, the actual pressure at the actuator must first fall below the desired pressure by a significant amount and remain below the desired pressure for a period of time before any movement of the pump's displacement control valve is initiated. Further, movement of the valve, because it is initiated primarily by the pressure differential across the valve itself, may not provide consistent operation under varying conditions (e.g., under varying temperatures and fluid viscosities). Further, pilot-type valves may exhibit instabilities in some situations because of their slow response time, the instabilities reducing the accuracy of the pump's displacement control.
An attempt to improve pump displacement control is described in U.S. Pat. No. 6,374,722 (the '722 patent) issued to Du et al. on Apr. 23, 2002. Specifically, the '722 patent describes an apparatus for controlling a variable displacement hydraulic pump. The apparatus includes a control servo operable to control an angle of the pump's swashplate, an electro-hydraulic servo valve connected to the control servo, and means for controlling the servo valve as a function of the pump's discharge pressure, as monitored by a discharge pressure sensor. Working on the principle of a negative feedback loop, the control servo is capable of sensing its actual position and comparing the actual position with an intended position that is associated with a desired discharge pressure. If the control servo detects a difference between the intended position and the actual position, the servo valve is energized to adjust the position of the control servo until the intended position is reached. In this way, the built in negative feedback loop of the control servo allows for very precise manipulation of the swashplate angle.
Although the apparatus of the '722 patent may help increase precision regulation of pump displacement, certain disadvantages may still persist. For example, the apparatus may not account for flow forces acting on the valve during operation of the pump. As such, displacement accuracy and response time of the apparatus may still be less than desired.
The disclosed hydraulic control system is directed to overcoming one or more of the disadvantages set forth above and/or other problems of the prior art.